I. Field of the Invention
The present invention relates to a method and apparatus for image processing which reduce redundancy of an image data source for transmission and which realize high speed image transmission without degradation in image quality.
II. Description of the Prior Art
Two high speed image transmission systems are conventionally known: the data source encoding system which realizes high speed transmission by reducing redundancy of the data source to thereby reduce the amount of image data to be transmitted per page, and the modulation/demodulation system which realizes high speed transmission by effectively utilizing the frequency band width of the transmission line to increase the amount of image data which may be simultaneously transmitted.
The M.H encoding system adopted in GIII standards of CCITT is an example of the former system, and the AM-PM-VSB modulation/demodulation system adopted in GII standards or the AM-DSB system adopted in GI standards is an example of the latter system.
In accordance with the known white block skip system a sequence of picture signals for a line is divided into blocks which are examined for black signals. If a black signal is not contained in a block, instead of the picture signals of this block, a skip signal consisting of fewer bits than the picture elements of the block is modulated and transmitted. For a block which does contain a black signal, the picture signals are modulated and transmitted. With this system, since the skip signals are scattered in the transmission signal sequence for a line, three kinds of signals, that is, the skip signal, the synchronizing signal, and the picture signal, must be correctly discriminated. For this reason, signals which have special characteristics never encountered in the picture signal sequence and which may thus be discriminated with certainty are used for the synchronizing signal and the skip signal.
In accordance with a known transmitting and receiving system combining the AM-PM-VSB system and the white block skip system, the synchronizing signal and the skip signal have amplitudes greater than the maximum amplitude (the white signal level in this case) which may be obtained in the picture signal sequence, and the synchronizing signal and the skip signal have phase data which may be differentiated from each other.
With such a conventional system, an amplitude control circuit and a phase control circuit are required at the transmission side, and an amplitude discriminating circuit and a phase discriminating circuit are required at the receiving side. This is disadvantageous in that the manufacturing cost of the equipment is high and the algorithm of transmission and reception is complex. In addition, since this conventional system also utilizes phase data, the white block skip system and the AM-DSB system may not be combined.